Lost circulation is defined as the total or partial loss of drilling fluids or cement into highly permeable zones, cavernous formations, and natural or induced fractures during drilling or cementing operations. The consequences of lost circulation can include:                Blowout, due to drop in fluid level in the well (loss of hydrostatic head);        Stuck pipe due to poor cutting removal;        Zonal isolation failure due to insufficient cement filling;        Increased cost due to loss of drilling fluids or cement, increased rig time and remedial cementing operations;        Formation damage due to losses to the producing zone; and        Loss of the well.        The severity of lost circulation can vary from minor (<10 bbl (1.5 m3)/hr) to severe (total loss of fluids, unable to keep hole full or obtain returns to surface).        
One approach to dealing with lost circulation is to add materials “lost circulation materials” or “LCMs”) to the fluid which bridge or block seepage into the formation. LCMs generally fall into four main types:                Granular (e.g. ground nut shells, plastics, or limestone);        Lamellar (e.g. cellophane flakes);        Fibrous (e.g. sawdust, hay, glass fibers); and        Encapsulated fluid-absorbing particles.        
LCMs can vary in size from 200 mesh to ¾ inch and are typically used at concentrations of from 8-120 lb/bbl according to the severity of losses.
Cement plugs, often including LCMs, set at the level of lust circulation have also been used to address these problems during drilling. Low density cements, including foamed cements have been considered as particularly useful. It has generally been considered that the use of LCMSs in cement slurries in only effective for minor or partial losses, and the for total loss situations, foamed cement is the only effective solution. The most common LCMs used in cement slurries have been granular materials such as Gilsonite, crushed coal, or ground nut shells. Cellophane flake material has been attempted but problems are encountered with mixing of the slurry at higher loadings. Fibrous materials are seldom used in cement slurries because fo problems in plugging the cementing equipment. One system using fibers that has been proposed is described in EP 1284248 and comprises the use of glass or polymer fibers in a low density cement slurry having solid materials present in discrete particle size bands.
Other gelling or viscous systems that do not contain Portland cement have also been used as plugs. Examples of these are gelling agents such as silicates with a suitable activator. Such plugs may also contain bridging materials such as ground calcium carbonate with particle sizes in the range 8 μm to 254 μm and at concentrations of up to 10 lb (4.5 kg)/bbl. Other gelling systems include Sorel cement (magnesium oxide, magnesium chloride and water).
Certain downhole-mixed system have also been proposed. These include mud-diesel-oil-bentonite (M-DOB) plugs and polysaccharide gelling systems encapsulated in emulsions that are broken by shear forces downhole (see EP 0738310). A development of this later system has the gelling system combined with cement (see WO 00/175481)
Further details of lost circulation problems and possible solutions can be found in Baret, Daccord and Yearwood, Well Cementing, Chapter 6 “Cement/Formation Interactions”, 6-1 to 6-17.
It is an object of the present invention to provide techniques that can be used during drilling to reduce problems associated with lost circulation.